Some of the initial concepts of coupling peptides or polypeptides to poly(ethylene glycol) PEG and similar water-soluble poly(alkylene oxides) are disclosed in U.S. Pat. No. 4,179,337, the disclosure of which is incorporated herein by reference. Polypeptides modified with these polymers exhibit reduced immunogenicity/antigenicity and circulate in the bloodstream longer than unmodified versions.
To conjugate poly(alkylene oxides), one of the hydroxyl end-groups is converted into a reactive functional group. This process is frequently referred to as “activation” and the product is called an “activated poly(alkylene oxide)”. Other substantially non-antigenic polymers are similarly “activated” or functionalized.
The activated polymers are reacted with a therapeutic agent having nucleophilic functional groups that serve as attachment sites. One nucleophilic functional group commonly used as an attachment site is the ε-amino groups of lysines. Free carboxylic acid groups, suitably activated carbonyl groups, oxidized carbohydrate moieties and mercapto groups have also been used as attachment sites.
Insulin and hemoglobin were among the first therapeutic agents conjugated. These relatively large polypeptides contain several free ε-amino attachment sites. A sufficient number of polymers could be attached to reduce immunogenicity and increase the circulating life without significant loss of biologic activity.
Excessive polymer conjugation and/or conjugation involving a therapeutic moiety's active site where groups associated with bioactivity are found, however, often result in loss of activity and thus therapeutic usefulness. This is often the case with lower molecular weight peptides which have few attachment sites not associated with bioactivity. Many non-peptide therapeutics also lack a sufficient number of attachment sites to obtain the benefit of polymeric modification.
One suggestion for overcoming the problems discussed above is to use longer, higher molecular weight polymers. Depending on the molecular weight desired, however, these materials can be difficult to prepare and expensive to use. Further, they sometimes provide little improvement over more readily available polymers.
Another alternative suggested is to attach two strands of polymer via a triazine ring to amino groups of a protein. See, for example, Enzyme, 26, 49–53 (1981) and Proc. Soc. Exper. Biol. Med., 188, 364–9 (1988). Triazine, however, is a toxic substance which is difficult to reduce to acceptable levels after conjugation. In addition, triazine is a planar group and can only be double-polymer substituted. The planar structure rigidly locks the two polymer chains in place. This limits the benefits of polymer conjugation to about the same as that obtained by increasing polymer chain length. Thus, non-triazine-based activated polymers would offer substantial benefits to the art.
In the above-mentioned cases, however, the biologically active polymer conjugates were formed having substantially hydrolysis-resistant bonds (linkages) between the polymer and the parent biologically-active moiety. Thus, long-lasting conjugates which are permanently linked rather than prodrugs per se (where the parent molecule is eventually liberated in vivo) were prepared.
Commonly assigned U.S. Pat. Nos. 5,643,575, 5,919,455 and 6,113,906 disclose additional improvements relating to multiple-strands of PEG sharing a common point of attachment to a nucleophile via an aliphatic linker. Unlike the earlier triazine-based branched polymer conjugates, the aliphatic linkers allow the artisan to avoid the toxicities of triazine as well as provide other useful advantages.
In addition, over the years, several methods of preparing prodrugs have also been suggested. Prodrugs include chemical derivatives of a biologically-active parent compound which, upon administration, will eventually liberate the active parent compound in vivo. Use of prodrugs allows the artisan to modify the onset and/or duration of action of a biologically-active compound in vivo. Prodrugs are often biologically inert or substantially inactive forms of the parent or active compound. The rate of release of the active drug is influenced by several factors including the rate of hydrolysis of the linker which joins the parent biologically active compound to the prodrug carrier.
Some prodrugs based on ester or phosphate linkages have been reported. In most cases, the particular type of ester linkage used to form the prodrug provides t1/2 for hydrolysis of up to several days in aqueous environments. Although one would expect a prodrug to have been formed, most of the conjugate is eliminated prior to sufficient hydrolysis being achieved in vivo. It would therefore be preferable to provide prodrugs which have a linkage which allows more rapid hydrolysis of the polymer-drug linkage in vivo so as to generate the parent drug compound more rapidly.
Prodrugs based on amide or carbamate linkages have also been reported. In general, amide bonds are known to be highly resistant to hydrolysis. However, it has recently been found that the C-terminal amides of ε-amino acids are readily hydrolyzed at 25° C. and pH 7.4 when the N-terminus is N-hydroxyethylated with one or two hydroxyethyl groups. Bis N-2-hydroxyethyl glycine (bicine) type molecules are key to such hydrolysis reactions. Such bicine type groups have recently been employed in the synthesis of prodrugs, see commonly assigned U.S. patent application Ser. No. 10/218,167, the contents of which are incorporated herein by reference.
There is still room for improvement in the area of prodrug design. The present invention provides such an improvement.